Injection molded microlenses for optical interconnects

ABSTRACT

Disclosed are a microlens array, and a method of positioning and aligning the microlens array on another device. Generally, the microlens array comprises an array of injection molded microlens elements, and a supporting flange. Each of the microlens elements has a generally spheroid or spherical shape, and the supporting flange connects together the array of microlens elements to facilitate positioning that array of lenses on a printed circuit board, semiconductor package or wafer. This array is well suited for use with vertical cavity surface emitting lasers (VCSELs); and, in particular, the preferred embodiment of the invention addresses the problem of VCSEL laser array alignment by using arrays of microlenses elements fabricated by injection molding.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/195,150,filed on Aug. 2, 2005, the disclosure of which is herein incorporated byreference. This application is related to co-pending application Ser.No. 11/195,147, filed herewith for “Injection Molded MicroOptics,” thedisclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to microlenses, and more specifically,the invention relates to injection molded microlenses that areparticularly well suited for optical interconnects.

2. Background Art

There has been recent interest in the development of higher bandwidthfiber optic interconnects for a variety of server and storageapplications. For example, optical transmitter arrays comprised ofvertical cavity surface emitting lasers (VCSELs) are commerciallyavailable, with up to 12 lasers per array on a 250 micron pitch. Thesedevices are interconnected with similar arrays of photodetectors usingribbons of optical fiber to form parallel optical interconnects (POI).These devices are available from a number of companies, includingAgilent, Tyco, Emcore, Picolight, and Xanoptix. These devices may beused, for example, on high end technical computers as part of theclustering fabric between switches; this enables higher bandwidth, andlonger distance links, which are essential to building large server,clusters. Smaller versions of POI, with only 4 optical elements perarray, are also used in high volumes for I/O applications, and otheruses are being developed for future clustering applications of thistechnology. Various widths of POI have been standardized, including 4X,8X, and 12X arrays at data rates ranging from 2.5 Gbit/s/line to 5Gbit/s/line. More advanced applications are also under consideration,including direct integration of VCSEL arrays into dual chip andmulti-chip modules.

POI offers many technical advantages, including significantly higherbandwidths and many times the distance of copper links; immunity toelectromagnetic interference; smaller, denser packaging; and lighterweight, more flexible cable assemblies. A significant inhibitor to thewider adoption of these links has been the relatively high cost comparedwith copper alternatives; consequently, POI is only used today inapplications which are either insensitive to cost or which require acombination of distance and bandwidth that cannot be achieved any otherway. Cost reductions for POI Would thus be highly desirable.

A major cost component in POI is the active alignment required betweenan array of lasers and a corresponding array of optical fibers. It hasbeen estimated that fabrication of such microlens elements is currentlya $1 B market opportunity, and growing larger in the coming years. Thegoal is to launch infrared radiation (typical wavelengths near 850 nm)from the VCSEL aperture (initially 2-3 microns diameter) into the fibercore (typically 50 microns diameter) as efficiently as possible with thelowest cost. Positioning the fiber core directly against the laseraperture (butt coupling) is not practical because the laser beam from aVCSEL has very high divergence; it is not possible using standardoptical array connectors (such as the MPO) to bring the fiberssufficiently close to the laser aperture. Even if this was possible andthe beam diameter was smaller than the fiber core, losses would stilloccur due to a mismatch with the fiber's numerical aperture (part of thebeam may still exceed the acceptance angle of the fiber and would not beguided).

An object of this invention is to enable wafer-scale manufacturing andelectronic integration with optical components.

For this reason, all practical VCSEL arrays employ some form of lensstructure to facilitate coupling light into the fiber array. Thisproblem is significantly more complex than alignment of a single laserand fiber, due to effects such as cumulative tolerance runout in thelens and VCSEL designs. Conventional lens elements can be fabricatedseparately (for example, spherical or ball lenses made of glass), thenmanually aligned with elements in the VCSEL array; this is not a lowcost manufacturing process, and uniformity of the coupled optical poweris not well controlled across the array. There is also highmanufacturing fallout from the failure to properly align lasers andlenses, or the failure of a laser array element after alignment, whichis an important reason for the high costs encountered today. Thus, thereis an industry need for a low cost, high volume fabrication method forVCSEL array lenses, and a low cost assembly/alignment procedure forattaching these lenses into a VCSEL package.

SUMMARY OF THE INVENTION

An object of this invention is to enable printed circuit board andwafer-scale manufacturing and electronic integration with opticalcomponents.

An object of this invention is to provide an improved microlens array.

Another object of the present invention is to provide a low costalignment procedure for attaching micro lenses on a printed circuitboard or semiconductor package or wafer.

A further object of the invention is to address the problem of VCSELlaser array alignment using arrays of microlens elements fabricated byinjection molding.

These and other objectives are attained with a microlens array, and amethod of positioning and aligning the microlens array on anotherdevice. Generally, the microlens array comprises an array of injectionmolded microlens elements, and a supporting flange. Each of themicrolens elements has a generally conic cross-section surface ofrevolution, which may be spheroidal, ellipsoidal, or cylindrical shape,and the supporting flange connects together the array of microlenselements to facilitate positioning that array of lenses. This array iswell suited for use with vertical cavity surface emitting lasers(VCSELs); and, in particular, the preferred embodiment of the inventionaddresses the problem of VCSEL laser array alignment by using arrays ofmicrolenses elements fabricated by injection molding.

The preferred embodiment of the invention allows for fabrication of lensarrays in a single process step, including features such asanti-reflection facets on the lens assembly. The molding process may bemodified to allow fabrication of flanges between lens elements with thesame high tolerances as the lens surfaces. The entire lens array may bealigned with lithographic precision to the VCSELs, eliminating some ofthe most time-consuming and costly steps in the current art. Thisprocess also enables wafer-scale test of the resulting VCSEL array,further reducing cost and improving yield. Additional molding featuresand additional uses for molded microlenses are described in copendingpatent application Ser. No. 11/195,147 for “Injection MoldedMicrooptics,” filed herewith, the disclosure of which is herebyincorporated herein by reference in its entirety.

In another aspect of this invention, a method is provided for forming adiffractive lens structure. This method comprises the steps of providinga mold plate, patterning a set of rings arrayed across the mold plate,and opening spaces between the rings, wherein the rings become anin-situ mask on the mold plate. The method comprises the further stepsof directing an optical polymer into the spaces between the rings,polymerizing said optical polymer to form an array of ring lenses, andtransferring the array of ring lenses to a substrate.

Preferably, the providing step includes the step of providing the moldplate with a layer of photoresist deposited thereon; the patterning stepincludes the step of using a photolithography system including a mask ofalternating dark and light rings, which correspondingly transmit andocclude light, to pattern the set of rings arrayed across the moldplate. Also, with the preferred embodiment, the directing step includesthe step of injecting the optical polymer into the spaces between therings, the opening step includes the step of developing and washing thepatterned arrays to open said spaces between the rings. This preferredmethod may also comprise the further steps of applying a release layerto conformally coat the sidewalls of the ring-etched structure, andforming alignment marks on the mold plate to facilitate aligning themold plate to said substrate.

Further benefits and advantages of the invention will become apparentfrom a consideration of the following detailed description, given withreference to the accompanying drawings, which specify and show preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical cavity surface emitting lasers (VCSELs) withwhich this invention may be used.

FIG. 2 illustrates an assembly for molding an array of microlenses.

FIG. 3 is a process diagram generally outlining a procedure for moldingan array of microlenses and then transferring the lenses onto asemiconductor wafer.

FIG. 4 illustrates a procedure for aligning and clamping a microlensarray to a semiconductor wafer.

FIG. 5 shows a procedure for inspecting a semiconductor wafer that isprovided with an array of microlenses.

FIG. 6 shows a process for forming an injection molded diffractive lensstructure.

FIG. 7 illustrates a cross sectional view of a diffractive lens formedusing the process of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention, generally, relates to a microlens array, and to amethod of positioning and aligning the microlens array on anotherdevice. Generally, the microlens array comprises an array of injectionmolded microlens elements, and a supporting flange. Each of themicrolens elements has a generally spheroidal, ellipsoidal, orcylindrical shape, and the supporting flange connects together the arrayof microlens elements to facilitate positioning that array of lenses.This array is well suited for use with vertical cavity surface emittinglasers (VCSELs); and, in particular, the preferred embodiment of theinvention addresses the problem of VCSEL laser array alignment by usingarrays of microlens elements fabricated by injection molding.

Since VCSELs emit an approximately circular cross-section beam, acircular symmetric lens or an anamorphic lens may be used in thisapplication. A typical VCSEL beam with 15-20 degrees divergence needs tobe coupled into a fiber core with an acceptance angle as low as 6degrees. However, the resulting optical subassembly must also be eyesafe, compliant with international laser safety regulations (IEC 825).The preferred laser product classification is Class 1, or inherentlysafe for viewing by untrained personnel without the aid of magnifyingoptics. One way to achieve this is by using the lens to controldivergence of the beam, which reduces the energy density of theaccessible laser radiation reaching the eye from an open VCSELtransmitter port. Partially collimating the beam with a microlens offersthe ability to maximize coupled power without sacrificing eye safety.The curvature of the lens should allow placement in the near field ofthe VCSEL to minimize effects of VCSEL divergence variations. Theseprinciples are illustrated by the attached figures.

More specifically, FIG. 1 illustrates a VCSEL 10, a pair of lenses 12,14 a partially collimated laser beam 16, and an optical fiber,represented at 20. In the arrangement of FIG. 1, the lens element 12 isattached to a support flange 22 in this example; the tolerances of theflange inner diameter is preferably very low in order to facilitatecoupling of the lens arrays. The use of a flange 22 such as thisfacilitates assembly of the microlens array inside the same package asthe VCSEL array, forming an integrated optical transceiver subassembly.The tolerance of the flange 20 and lens elements 12, 14 causes opticalpower variations of 25% or more if the connector position is misalignedby less than 1 micron. Thus, the means used to fabricate the microlensarray preferably preserves low tolerances and minimizes array runout.Because of their small size, microlenses 12, 14 cannot be polished usingconventional methods; the fabrication process preferably results in asurface sufficiently smooth for optical applications. Finally, themicrolens preferably minimizes back reflections of light into the laser,which causes instabilities known as reflection-induced intensity noiseand degrades bit error rate. A slight flattening 24 at the lens apex, ata shallow angle (4-6 degrees), may be used to minimize back reflectedlight.

With the preferred embodiment of the present invention, VSCEL lensarrays having these properties are fabricated using an injection moldingtechnique, as illustrated in FIGS. 2 and 3. With reference to theseFigures, first, as represented at 30, a metal molding plate 32 such asmetal, silicon graphite, glass, etc., is designed with an array ofcavities 34 that have the desired radius of curvature for the VCSELmicrolens system. If the mold plate is used for lower temperature lensmaterials, it may be fabricated out of silicon. The mold plate 32 can bedesigned with an arbitrary surface curvature in the cavities,facilitating the design of partially collimating lenses for laser eyesafety. The resulting lens elements will be closely spatially matched tothe dimensions of the VCSEL elements; variations in the lens spacing orcurvature facilitates spatial transforms to other fiber pitches, as wellas the use of different fiber core diameters/numerical apertures.

As represented at 36, this mold plate 32 is filled with a suitable lensmaterial 40, such as low temperature (in the range from 100° C. to 150°C.) glass or plastic, which has a moderately low melting point (in therange from 150° C. to 250° C.) and suitable refractive index (in therange from 1.3 to 3.3). Using selected wavelength(s) of actinicillumination, photopolymerization to a specific degree enables tuningthe refractive index of the microlens array, either in situ in the moldcavities or post-transfer to the target device array. Alternately, withspecial processing steps any type of higher melting point glass could beaccommodated for additional cost. Depending on the material choice, themold may first be coated with an optional release layer (such as Teflon)to facilitate de-molding of the microlenses; in the case of microlensarrays, the top surface of the substrate may also be coated with arelease layer. Note that a release layer may also be applied to the lensflange region to facilitate removal of the lens array. The mold plate 32is filled with molten lens material; preferably, the lens flange 22connecting multiple array elements is fabricated at the same time byleaving a connection 42 in the mold plate between adjacent lens elements(which may also be coated with the Teflon release layer). This providesfor tight control of the flange tolerances. Alternately, as representedat step 44 in FIG. 3, the VCSEL surface may be molded or machined in aseparate operation to form a conjugate, conformal surface which wouldmate with the flange 22 to facilitate passive self-alignment of theVCSEL and lens array in the near field of the VCSELs.

The pressure feed for the lens material 40 may need to be regulated inorder to uniformly fill both the deep lens cavity 34 and the shallowconnecting flange 42. To facilitate this, it may be desirable tomaintain the lens material melt at a sufficiently high temperature (inthe range from 150° C. to 300° C.) so that it remains fluid until anentire lens array is filled, rather than beginning to cool before theentire array is complete (which may lead to thermal imbalance andcracking); an optional heating element 46 is shown wrapped around theinjection tool 50 for this purpose. Alternately, the speed of the scanof the injection tool across the mold plate 32 may be varied so that theinjection tool passes more quickly over thinner areas of the substrate(such as those along the flange or between the lens elements). Anoptional digital microcontroller (not shown) may be interfaced with ascan position controller for this purpose. The mold cavity 34 can bedesigned with a slight flattening 52 at the lens apex and a shallowangle, to minimize back reflected light. Alternately, the lens apex mayundergo reactive ion etching (RIE) to fabricate this feature.

The mold plate 32 preferably contains alignment markings to facilitatepositioning and removal of the microlens arrays. Complementary markingsare provided on the VCSEL wafer, etched into the substrate at either endof the other array or otherwise provided; these markings may beimplemented with lithographic precision. The alignment of the VCSELarray to the lens array, represented at 54 in FIGS. 3 and 4, may be doneusing an optical inspection technique under visible light; there is norequirement for manual adjustment of the lens array. The processinvolves inverting or flipping the lens array onto the VCSEL wafer;inspecting & aligning the markings on the lenses and VCSELs; clamping orholding in fixed position the mold plates in the aligned position, asrepresented at 56; transferring 60 the microlens array onto the VCSELsurface, separating the lens/flange release layer, as represented at 62;and, as represented at 64, removing the mold plate for cleaning andreuse, leaving the lens array affixed in place with either opticalepoxy, index matching gel, or similar agents.

This enables wafer-scale testing 66 of the VCSEL-lens array. Withreference to FIG. 5, the lens elements 70 may be attached before theVCSEL wafer 72 is diced, and electrical probes are used to apply voltageto the wafer scale VCSEL elements and produce light output. The opticalproperties of the laser-lens array, sensed by any suitable sensor ordetector, represented at 74, may be characterized on an integrated waferscale, saving considerable cost in manufacturing test.

An alternate technique may be used for controlling high levels of laserlight back reflection. By slightly tilting the mold plate 32 while thelens material 40 is still melted, the top surface of the lens array canbe made to flow into a very shallow angle (a few degrees). If the moldis held in a tilted position until the lens material hardens, it ispossible to form an array with lenses on one side and a back-reflectionangle on the other side across the entire lens array. For applicationsin which high levels of back reflection are anticipated, or in whichback reflection would be much larger than the original transmitted beamaperture, this prevents excess light reflected off-axis of the microlensfrom returning to the laser aperture.

Additional molding features and additional uses for molded microlensesare described in copending patent application Ser. No. 11/195,147 for“Injection Molded Microoptics,” filed herewith, the disclosure of whichis hereby incorporated herein by reference in its entirety.

The flow-chart in FIG. 6 depicts the ordered process sequence forforming a Fresnel zone plate diffractive lens structure. At step 80, asuitable mold plate or template material, such as an optically flatborosilicate glass plate, has deposited on t a blanket layer ofphotoresist. At step 82, a projection photolithography system using amask of alternating dark-light rings, which correspondingly transmit orocclude light, photopatterns a set of concentric rings or annuli arrayedacross the template. At step 84, subsequent development and washing ofthe patterned array, opens the inter-ring spaces. The rings are now anin-situ mask for reactive ion etching, RIE, or hydrofluoric acid orother suitable wet-etching, of the mold plate, which is done at step 86.To assure detachment of the injection molding optical polymers of lowmelting point (less than 300° C.) glasses which will be used to fill thering-cavity array, a release layer is, at step 90, applied toconformally coat the sidewalls of the concentric ring etched structure.At step 92, the fill tool then injects the optical polymer into thering-cavities. Photolithographic alignment keys are concurrently formedduring the ring patterning and etch steps, positioned appropriately onthe mold plate for optimum alignment to conjugate alignment markslocated in corresponding positions on the device wafer or substrate onwhich the injection-molded concentric ring arrays will be transferred.At step 94, alignment, clamping and transfer of the photopolymer ringlenses is then completed. At step 96, separation and subsequentillumination by a light source having a spectral content which will beabsorbed by the photopolymer will induce photopolymerization tocross-link the ring-lens material into a harder, more durable state,and, simultaneously can be used to fine tune the index of refraction ofthe diffractive lens or Fresnel.zone plate. In addition, the RIEconditions may be adjusted to achieve a degree of anisotropy byadjusting the RIE chamber pressure or gas composition to allow somefaceting or blazing of the ring surfaces comprising the diffractivelenses. FIG. 7 illustrates the cross-sectional view of the diffractivelens.

By applying the very precise technique of photolithographic alignmentusing fiducial marks, diffractive lenses may be overlayed directly ontoVCSEL junctions to achieve ideal high coupling efficiency in the laser'snear field. The present invention thus teaches the wafer-scale alignmentand attachment of coupling either refractive or diffractive lens to theemitting junctions of VCSEL devices, and, is readily extended to similarattachments to MEMS device arrays of MEMS wafers, or, combinations ofVCSEL and MEMS configurations.

The fundamental Physical Optics condition to be satisfied to form a zoneplate is that the alternative transmissive and absorptive or reflectiverings have diameters such as to make a periodic function of the peaksand nulls resulting from a specified amplitude-phase interferogram. Suchconditions are well known to one skilled in the art and fully enabled bythe present invention and the controlled variations of parameters suchas radial step size, annular thickness, height of the annuli, blazeangle, refractive index, and reflectivity. A principal advantage of thepresent invention is the scalability of the diffractive microlens andits wafer-scale fabrication.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects stated above, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art and it is intended that the appended claims cover allsuch modifications and embodiments as fall within the true spirit andscope of the present invention.

1. A method of aligning a microlens array on a given device array,comprising the steps: forming said array of microlenses in a mold plate;positioning the mold plate with the lenses therein in a defined positionrelative to the given device; holding the mold plate and the givendevice together; separating said array from the mold plate; removing themold plate from said given device, leaving said array in place on saidgiven device array said method further comprising the step of testingoptical properties of said lens array with said lens array on said givendevice array, wherein the testing step includes the steps of: attachingelectrical probes to said given device array; applying a voltage to saidgiven device to produce light output; and using said light output tosense said optical properties of said lens array, said method furthercomprising the step of dicing said given device to form a multitude ofintegrated circuit chips; and wherein the testing step includes the stepof testing said optical properties before dicing said given devicearray, said method further comprising the step of providing the moldplate and said given device with alignment markings; and wherein thepositioning step includes the step of using said alignment markings toposition the mold plate in said defined position; and the separatingstep includes the step of affixing said lens array on the given devicearray, wherein the microlens array minimizes back reflections of lightinto a laser.